This invention relates to an intrinsically safe (IS) membrane or matrix keyboard. More particularly, this invention relates to the use of opto-couplers and a single IS barrier to thereby render a matrix keypad or keyboard intrinsically safe.
As is well known, in the electronic circuit design field, the term "Intrinsically Safe", means that the amount of power (voltage.times.current) or energy in use in a given circuit is less than that level of power required to ignite an explosive mixture of gas. The use of Intrinsic Safety (IS) as a method of circuit protection demands that every conductor (e.g. wire) entering a hazardous area be isolated so that the energy which is being transmitted into the hazardous area will be safe (e.g. below the level necessary for an explosion to occur).
Matrix or membrane keyboards have a well known structure and consist of an array of switches configured into a fixed number of rows and columns. Each individual row and column is connected to keyboard scanning circuitry by a separate wire. When the keypad is located in a hazardous environment, the keyboard scanning circuitry is located in a remote non-hazardous area with the separate wires providing the link between the keypad (hazardous area) and the keypad scanning circuitry (non-hazardous area). In order to render the keypad "Intrinsically Safe", a separate barrier must be used to protect each individual wire. These barriers are known as Intrinsically Safe barriers (hereinafter "IS barriers") and are commercially available units which utilize a transformer unit or Zener diode unit.
Unfortunately, the use of a separate IS barrier for each wire which runs into and out of the keypad leads to several important drawbacks. For example, a multiplicity of IS barriers results in a bulky and cumbersome approach to protecting the keypad. Moreover, the use of a plurality of IS barriers is quite expensive both in terms of purchasing and labor costs.